Vertical mounting of a HD will help, but not necessarily prevent damage. During high-G shocks, the flying write heads will slam into the storage platters, not only taking a chunk out of the iron oxide, it will also tend to beat the write head pretty good.

A further step (albeit not a complete solution), which is rearely taken by manufacturers (I'm not sure if ANY HD-based MP3 player out there uses it), is to read the HD in short bursts and store the data onto a flash memory card or RAM. The reading is done in short bursts that last a couple of seconds max, but enough to grab several minutes of music data. Any time the HD is not being read, the heads are parked off platter, so even if a shock does occur, no platter damage will happen to ruin your day.

 

I had an xbox in my wrx for a couple thousand miles. It held up really well. It's pretty much a computer isn't it? Hard drive and all?
In any case, my car was slammed to the ground on stiff Tein HA's, so it got pretty bumpy in there. DVDs played fine even on socal's nasty freeways. Machine (xbox) still works great to this day.
Oh i used a power inverter from best buy. It was in their video game section.

 

You can't really avoid shock in this car. It's a sportscar not a luxo-barge. What you can do though is use drives that are designed for hostile environments. Laptop drives have much higher G-ratings than PC drives because they are expected to get banged around. If you can use one in an iPod you can use it in a car.

Another alternative, albeit with limitations, is a solid state hard drives. You can get IDE to CF card boards and run an entire OS from it. Then store you MP3s on CD. I realise Winblows won't fit on any reasonable CF card but we have FreeBSD running on a 256Mb card with room to spare and youcan get 2Gb cards these days.

As for power, forget 120VAC. The first thing you see when you open up any of these devices is a chunky power supply to convert mains power to 12VDC. As this is the native power of the car why not use it directly. Infact the mini-ITX cases we use have an external 12VDC power supply so the 12VDC just plugs straight in (OK, through a noise suppressor and stiffening capacitor).